1. Field of the Invention
This invention pertains to sonic transmission systems and more particularly to systems for transmitting low frequency sonic signals with long range characteristics. Most particularly, this invention pertains to a parametrically pumped sonic weapon system.
2. Statement of the Prior Art
In order to destroy a target or kill or injure enemy personnel using a sonic signal, it is necessary to vibrate the target at or near its resonant frequency. Since the resonant frequency of commonly encountered structural targets is relatively low, typically 5-20 hertz, a low frequency sonic beam is required. In addition, the sonic beam must have sufficient range to be effective as a weapon. While these two criteria are simple to state, they are not easy to implement. The reason is Rayleigh's Law which may be written in equation form as follows: EQU .delta.=1.22 .lambda./sin.phi.
where
.delta. equals the radius of the central disc of energy,
.lambda. equals the wavelength of the focused beam and
.phi. equals the angle subtended by the lens at the focal distance. For example, Rayleigh's Law predicts that the lens diameter required to focus a 10 hertz wave to within a 50 foot diameter zone of focus at a range of one mile is about 26,484 feet or about five miles.
Until recently, Rayleigh's Law was thought to be an absolute bar to the long-range propogation of a low frequency wave. Recently, however, it has been recognized that if two colinear sound beams are introduced into a nonlinear transmission medium, the interaction between them results in the production of the difference frequency. Thus, if the frequency of one of the beams is f and the frequency of the other beam is 2f, then the difference frequency component will also be f and may be used to augment the lower frequency sound beam. Moreover, the difference frequency component will have the range characteristics of the higher frequency component. However, this augmentation, commonly referred to as parametric pumping, will only take place if the phase difference between the two signals at the starting point of the interaction is approximately 90.degree.. Otherwise, the component produced by the non-linear interaction will tend to oppose the lower frequency signal. The theoretical basis for these conclusions is set forth in an article by O. V. Rudenko and S. I. Soluyan entitled Theoretical Foundations of Nonlinear Acoustics (English translation) Consultant's Bureau, New York 1977, pp. 145-157. Applicant is not aware of any system which utilizes these principles to focus low frequency sonic signals over relatively large distances much less one that does so with sufficient power to destroy remote targets or kill or injure enemy personnel.